Geostationary artificial satellites (hereafter, also simply “satellites”) are used in services such as satellite broadcasting and satellite communications. Geostationary artificial satellites circle the earth along a predetermined geostationary orbit that has the same period as that of the earth's rotation. Viewed from the ground, thus, a geostationary artificial satellite appears to stand at the same elevation and azimuth at all times. In actuality, the position of the satellite deviates slightly from the geostationary orbit as a result of forces such as the gravitational pull of the sun/moon/earth, solar radiation pressure and the like that act on the satellite. During operation of the geostationary artificial satellite, therefore, it is necessary to correct periodically deviations from orbit. Specifically, the distance from the ground to the satellite is measured, to determine thereby the position of the satellite (this is referred to as “ranging”), and the position and attitude of the satellite are adjusted then through jet firing as needed. High-precision ranging is required in order to carry out such orbit control accurately and efficiently.
Various ranging methods have been proposed. One such known method (referred to as “one-station ranging”) involves measuring the distance from one ground station (antenna) to the satellite, as well as the elevation and azimuth. However, the distance from the ground station to the satellite is very large, and hence small angle errors exert a significant influence on position measurement precision within a plane that is perpendicular to the line that joins the ground station and the satellite. Accordingly, a problem arose in that, in order to achieve a practicable degree of precision with one-station ranging, substantial equipment costs are incurred on account of the need for an antenna that has a large diameter (for instance, about 5 m for radio waves in a 12 GHz frequency band) and that is equipped with a high-precision angle control mechanism.
Patent Literature 1 discloses a ranging method that utilizes a plurality of ground stations. This document, paragraph 0028; FIG. 5, discloses a method that involves arranging one transmission station and a plurality of reception stations, causing an uplink reference signal (time stamp) from the transmission station to be returned by the satellite, and measuring the time that elapses until the signal is received by each reception station, to pinpoint thereby the position of the satellite, in accordance with the principles of trigonometric or quadrilateral surveying. This document also discloses the feature of using a GPS (Global Positioning System) satellite for synchronization of times between reception stations. The method of Patent Literature 1 can be expected to afford better precision than one-station ranging, but has a drawback in that part of a transponder, which is to be used for a service, such a broadcasting or communications, must be utilized on account of a reference signal for ranging. Also, some equipment for uplink is required in order to transmit, to the satellite, the reference signal for ranging. In the case of a satellite (for instance, a spare satellite) for which the transponder that corresponds to the frequency band for ranging is not in use, moreover, a problem arises in that re-transmission of the reference signal is not possible, and ranging itself cannot be carried out.